This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-341120, filed Nov. 25, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus and an exhaust gas purifying method for an internal combustion engine, wherein a filter for collecting particulates exhausted from the internal combustion engine, and reactivating element for recovering the function of the filter are provided.
2. Description of the Related Art
As an example of an internal combustion engine, there is a diesel engine. The diesel engine has an exhaust gas purifying apparatus in the exhaust gas path. The exhaust gas purifying apparatus has a diesel particulate filter (DPF) and a reactivating unit. The DPF collects particulates such as black smoke, soot, HC, and the like, which are included in the exhaust gas. The recovery unit is disposed further upstream than the DPF in order to maintain the function of the DPF. The recovery unit has an oxidation catalyst, and NO and O2 included in exhaust gas are made react with each other, and NO2 is generated. The generated NO2 reacts with the particulates. As a result, the particulates collected at the filter are eliminated, and the DPF is recovered.
The particulates react with O2 at an ambient temperature of about 550xc2x0 C. or more, and react with NO2 at an ambient temperature of about 250xc2x0 C. or more. In a case of a continuous operating state such that the engine is maintained to have a constant number of revolution or more, a temperature of exhaust gas is maintained at about 250xc2x0 C. or more. In this continuous operating state, provided that NO2 is supplied by the recovering unit, the DPF can realize the so-called continuous recovering state in which particulates are burnt while collecting the particulates.
However, if light load operation continues, the temperature of the exhaust gas falls. As a result, because it is difficult to maintain the temperature of the oxidation catalyst disposed further upstream than the DPF at an activation temperature, there are cases in which recovery of the DPF is insufficient. In the exhaust gas purifying apparatus, if the recovering function deteriorates, the DPF is in an over-collected state, and clogging by particulates arises. As a result, an exhaust gas pressure increases, and the fuel efficiency and the power performance of the engine deteriorate.
Then, a recovering unit in which, due to an operating state of an engine being controlled, an amount of NO2 is increased by increasing an exhausted amount of NO, or reaction between NO2 and particulates is promoted by raising a temperature of exhaust gas, is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2001-115822. The recovering unit estimates an amount of the accumulated particulates collected at the DPF by one of an exhaust gas sensor, an engine speed sensor, an engine load sensor, an engine operating timer, an intake air amount sensor, an NOx sensor, an O2 sensor, and an exhaust gas temperature sensor. The recovering unit controls one of the fuel injection period and injected amount of a fuel injector, the opening of an EGR valve of an EGR (Exhaust Gas Re-circulation) device, the intake air amount of a turbosupercharger, the opening and closing periods and lift amounts of an intake air valve and an exhaust gas valve which are provided at the respective cylinders, and the opening of an intake air throttle valve provided in an intake air path.
Further, an exhaust gas particulate purifying apparatus having fuel injecting means for injecting fuel to the upstream side of the DPF is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 7-259533. The exhaust gas particulate purifying apparatus has a filter whose catalyst is coated, a temperature sensor for detecting a temperature of the filter, and fuel injecting means for injecting fuel to the upstream side of the filter on the basis of a temperature signal of the temperature sensor. The exhaust gas particulate purifying apparatus determines a recovery time of the filter on the basis of the temperature detected at the temperature sensor, and controls an injection timing and amount of fuel. Further, the exhaust gas particulate purifying apparatus additionally injects fuel into a cylinder during the expansion stroke, and makes oxygen of excess air and fuel remaining in the cylinder react with each other, and raises the temperature of exhaust gas. The exhaust gas whose temperature has been raised warms the filter having the catalyst so as to be greater than or equal to the activation temperature of the catalyst. Then, the warmed catalyst oxidizes the additionally injected fuel. The particulates collected at the filter are burnt by heat of reaction at the time of oxidizing fuel. When a state in which the temperature of the filter is greater than or equal to an ignition temperature of particulates passes a predetermined time, fuel injection from the expansion stroke to the exhaust stroke is stopped, and the reproduction of the filter is completed.
However, when the so-called forcible recovery is executed by raising the temperature of exhaust gas due to the load of the engine being varied such that the oxidation catalyst becomes an activation temperature in order to recover the filter, a larger number of particulates are included in the exhaust gas as compared with exhaust gas under normal operating conditions. Then, the particulates generated during the time until the oxidation catalyst reaches the activation temperature adhere to the oxidation catalyst. When the forcible recovery is repeated, the oxidation catalyst is gradually covered with particulates, and the function thereof cannot be sufficiently performed. As a result, in the exhaust gas purifying apparatus, the function of continuously recovering a filter deteriorates.
An exhaust gas purifying apparatus for an internal combustion engine which can improve a forcible recovery function while maintaining the continuous recovery function of a filter for collecting particulates included in exhaust gas of the internal combustion engine, has been desired.
An exhaust gas purifying apparatus for an internal combustion engine according to the present invention comprises a particulate filter, an oxidation catalyst, and forcible recovering control element. The particulate filter is disposed in an exhaust gas path, and collects particulates in exhaust gas. The oxidation catalyst is disposed in the exhaust gas path in further upstream than the particulate filter. At the time of forcibly recovering the particulate filter, after the forcible recovering control element executes catalytic temperature raising control in which the catalyst is activated by rising an exhaust gas temperature of the internal combustion engine, the forcible recovering control element executes filter temperature raising control in which the temperature of the filter is raised by supplying unburned fuel to the oxidation catalyst.
The exhaust gas purifying apparatus further comprises a front oxidation catalyst, a bypass path, and a passage switching device. The front oxidation catalyst is disposed in the exhaust gas path further upstream than the oxidation catalyst. The bypass path is provided in the exhaust gas path so as to bypass the front oxidation catalyst. The passage switching device switches the flow of exhaust gas to the front oxidation catalyst side or the bypass path side. The passage switching device switches the flow of exhaust gas to the front oxidation catalyst side at the time of executing the catalytic temperature raising control, and switches the flow of exhaust gas to the bypass path side at the time of executing the filter temperature raising control.
When the filter is forcibly recovered, at the time of executing the catalytic temperature raising control, the passage switching device switches the flow of exhaust gas to the front oxidation catalyst side. The front oxidation catalyst positioned further upstream than the oxidation catalyst is raised in temperature for a short period, and a rise of the temperature of the exhaust gas flowing at the oxidation catalyst downstream is promoted by heat of catalyst reaction. The oxidation catalyst is efficiently activated, and the soot exhausted from the engine is oxidized and burnt by the front oxidation catalyst which becomes a relatively high temperature accompanying a rise of the temperature of exhaust gas of the engine. Accordingly, at the same time when the soot can be prevented from adhering to the oxidation catalyst, activation of the oxidation catalyst can be further promoted by heat due to the soot being burnt.
Thereafter, at the time of executing the filter temperature raising control, the passage switching device switches the flow of exhaust gas to the bypass path side. Accordingly, the exhaust gas purifying apparatus can reliably burn unburned fuel by the oxidation catalyst, and can efficiently raise the temperature of the filter and efficiently recover the filter.
As a preferred mode of the invention, the front oxidation catalyst may be structured so as to have a capacity smaller than that of the oxidation catalyst downstream. In this case, at the time of executing the catalytic temperature raising control, the smaller the capacity of the front oxidation catalyst, the shorter the time in which the temperature of the front oxidation catalyst is raised to a sufficient temperature.
Further, a passage switching device of another preferred mode makes exhaust gas flow to the bypass path side by switching the flow other than at the time of the catalytic temperature raising control, in order to prevent the exhaust pressure of the internal combustion engine at the time of normal operation from rising. In order to efficiently execute forcible recovery of the filter, a temperature sensor may be provided between the oxidation catalyst and the filter. It is preferable as well that a temperature of the inlet of the filter is detected by the temperature sensor, and the passage switching device is operated by the forcible recovering control element on the basis of the detected temperature. Moreover, in order not to change the rate of flow and the pressure of exhaust gas, it is preferable as well that a cross-sectional area of the passage of the front oxidation catalyst and a cross-sectional area of the passage of the bypass path are determined in consideration of the pressure loss. The passage switching device may have valves on the respective front oxidation catalyst side and bypass path side, and the valves may be respectively structured so as to be separately openable and closable.
An exhaust gas purifying method for an internal combustion engine according to the present invention comprises: by using the above-described exhaust gas purifying apparatus, a step of switching, by the passage switching device, the flow of the exhaust gas to the front oxidation catalyst side at the time of executing the catalytic temperature raising control; and a step of switching, by the passage switching device, the flow of the exhaust gas to the bypass path side at the time of executing the filter temperature raising control. Further, the exhaust gas purifying method for an internal combustion engine according to the invention may comprise a step of switching, by the passage switching device, the flow of the exhaust gas to the bypass path side other than at the time of executing the catalytic temperature raising control.